The present invention relates generally to an improved apparatus for recording an image on a recording medium, and particularly to an optical system for use in a so-called flying spot scanning system (hereinafter referred to as "flying spot scanner").
There have been several proposals for the improvement of the flying spot scanner. One of them is shown in U.S. Pat. No. 4,034,408, wherein a laser beam emitted from a light source is modulated by an optical modulator in response to image signals. The laser beam is then deflected by a multifaced rotating polygon and condensed by an f-theta lens which is provided immediately after the polygon, whereby a recording spot of the laser beam scans a recording medium which is fed in a direction perpendicular to the direction in which the recording spot runs, to record a desired image on the recording medium.
Another proposal is shown in a publication "KOGAKU" (Japanese Journal of Optics) Vol. 6, No. 2, pp. 67-74, entitled "The Correcting Method of Beam Scanning Distortion Using the Parabolic Mirror". In order to correct distortion on a recording plane, a laser beam emitted from a light source is first expanded by a beam expander, then deflected by a galvanometer mirror and in turn reflected by a parabolic mirror which is directed toward a recording medium, whereby the recording spot of the laser beam scans the recording medium, to record a desired image thereon.
Recently, there has been a demand for a system which can record an image onto a recording medium of relatively large size, (e.g. having an effective recording width of approximately 500 mm to 1000 mm) with high resolution. Such recording requires a recording spot with a fine diameter, e.g. up to approximately 3 .mu.m to 10 .mu.m at half-width power thereof. However, conventional apparatuses cannot meet such demands satisfactorily. The reasons are discussed hereinbelow.
Generally, when a conventional apparatus is used to record on a large-sized recording medium, large undesirable aberrations are caused. The diameter of a recording spot W.sub.o can be expressed by: EQU W.sub.o =k.multidot..lambda..multidot.F (1)
where k is a constant, .lambda. is a wave length and F is an F-number.
To reduce the diameter of the recording spot W.sub.o in the apparatus illustrated in U.S. Pat. No. 4,034,408 to approximately 3 .mu.m to 10 .mu.m, an f-theta lens having a small F-number would have to be prepared, since both k and .lambda. are constant. It will be impractical, though not impossible in a theoretical sense, to obtain such an f-theta lens which meets the conditions required. It could, of course, be possible to correct the aberrations by using only a paraxial region of the f-theta lens. However, then the apparatus as a whole would be undesirably large because the optical path would be too long. This is apparently disadvantageous.
Furthermore, even if an f-theta lens which could satisfactorily reduce a recording spot to approximately 3 .mu.m to 10 .mu.m, polygon having reflective surfaces large enough to scan a whole recording width of a recording medium would be required. Such a multi-faced rotating polygon would have to have a diameter of 500 mm and would have to be controlled at a predetermined rotation rate so as to accurately scan the recording spot across a recording medium. This would be impracticable and costly.
With respect to the apparatus illustrated in the publication "KOGAKU", aberrations, particularly coma and astigmatism, cannot be corrected by means of a parabolic mirror. Accordingly, the diameter of the recording spot cannot be reduced to approximately 3 .mu.m to 10 .mu.m.